The invention relates to a method of identifying/monitoring active atherosclerotic plaques associated with blood vessel walls wherein the plaques comprise activated macrophages having accessible binding sites for a ligand. The method comprises the steps of administering to a patient being evaluated for atherosclerosis an effective amount of a composition comprising a conjugate of a ligand and a chromophore capable of emitting light under predetermined conditions, allowing sufficient time for the ligand conjugate to bind to the activated macrophages, subjecting the blood vessels to the predetermined conditions using a catheter-based device, and identifying active plaques by detecting light emitted by the chromophore using a catheter-based device or by using an external imaging technique. The invention also relates to a similar method wherein a chemical moiety capable of emitting radiation is conjugated to the ligand.